The Poggendorff illusion - dots as context elements

Abstract

The intensity of the classic Poggendorff phenomenon, i.e., the apparent shift of the protruding ends of a line crossing behind a bar, can be enhanced or reduced considerably by adding dots symmetrically to the edges of the vertical bar. Experiments show that this effect depends strongly on the position of the dots relative to the merging points of the target line. A conceptual approach is presented where this observation is related to more general laws on the perceived orientation of line segments as well as to a size constancy effect. An analytical expression is derived which is fitted to the experimental results. Finally an observation of the classic form of this illusion in a 16th century painting is reported.